I have an old house built in 1968. All the plumbing is galvanized and the backflow valve is from 1975. The PRV is a single chamber and may have been replaced once but looks to be 20+ years old. The issue I'm having is dirty water settling in the toilet tanks and when flushing the bowl gets dirty water. Also you have to run the kitchen sink for a 30 seconds to get really clear water that is not discolored. If I leave the house for 4 days and come back and go to the farthest bathroom and turn on the sink.. the water will be dirty. I already know the water heater has dirty water in the bottom and I have not flushed it yet. This is what I've been told by a plumber:

1. The PRV could have lots of corrosion causing the dirty water. Recommended changing it.
2. Flush the water lines and water heater.
3. Suspects that since the backflow valve was installed in 1975 it will be filled with all kinds of dirty muck built up.

The plumber said to replace all 3 and it should stop the dirty water issue.

He also said when looking at the galvanized piping that there were no physical indicators (moisture on the pipes) to make him think the issue is the pipes.

I've also been told by the water department that its the pipes. They said they ran the water from the spigot for 30 seconds and it cleared up. The PRV is under my house and the spigot is not reduced pressure.

My plan is to cut the nipple from the backflow valve and install the new dual chamber PRV at the road. The plumber Ive been talking to says he believes the source of the dirty water is the backflow valve and the muck in the line at that point. He said it happened to him at his house. So does anyone have any thoughts about this? Thanks